An electrical resistor is a passive two-terminal electrical component mainly characterised by its electrical resistance as a circuit element. Electrical resistors are ubiquitously employed in electronic circuits for dividing voltages and adjusting current intensity and signal levels, among other uses. Thus, the reliability and utility of an electrical resistor strongly depends on the accuracy of its electrical resistance value, that is, the precision to which the value of the true electrical resistance thereof, that can be measured, e.g. by means of an ohmmeter, coincides with a nominal electrical resistance value aimed at when manufacturing the electrical resistor.
Electrical resistors typically comprise an electrically resistive element extending between two electrically conductive terminations, wherein the value of the electrical resistance is determined by a cross-section of the electrically resistive element and its length extending between the two electrically conductive terminations as well as the conductivity of the electrically resistive material the electrically resistive element is made of. Imprecisions in the cross-section or the length of the electrically resistive element between the electrically conductive terminations may hence result in a deviation from the nominal value of the electrical resistance of the electrical resistor.
The electronics industry calls for electronic components of increasingly reduced size. This considerably adds to the technical complexity of the manufacturing of electrical resistors with a reliably determined electrical resistance value. Since the electrical resistance of an electrical resistor is closely related to the geometrical dimensions thereof, a precise control of the electrical resistance of an electrical resistor requires a highly accurate definition of its size during a manufacturing process thereof. However, the use of high precision methods for determining the size of electrical resistors at industrial level remains incompatible with the required production yields necessary for ensuring economic viability in the production.
A well-established solution relies on a combined use of less precise and less costly deposition processes for defining the basic structure of an electrical resistor, like for example screen printing, with a subsequent fine adjustment or trimming of the dimensions of the electrically resistive element between the electrically conductive terminals by means of more precise, though necessarily more technically involved and costly subtractive technologies, like laser ablation. According to this solution, a screen template or mask is employed for a preliminary formation of the electrically resistive element, whereupon the electrically conductive terminals are formed and laser trimming is used for accurately determining the shape and dimensions of the electrically resistive element, in particular its length extending between the electrically conductive terminals, and thereby fine tuning the final electrical resistance value of the electrical resistor.
An alternative to laser trimming as subtractive technique used for finely adjusting the dimensions of an electrically resistive element is disclosed in U.S. Pat. No. 6,225,035 B1, according to which an electrically resistive element is formed of a sensitive material allowing for subsequent subtractive treatment by means of photolithography.
While using subtractive methods of the type described above allows for obtaining an electrical resistance value with a desired precision, they tend to increase the manufacturing costs and manufacturing time. On the other hand, when using purely additive technologies, so far the achievable precision is not sufficient for many purposes, such as for example for use as an electrical pre-resistor of an LED.
Thus, there is room for technical improvement in the manufacturing of electrical resistors, in particular concerning the ability to guarantee a high degree of accuracy of the electrical resistance value while maintaining the technical and economic viability of the employed manufacturing methods.